Technical Feasibility Assessment for Roll Pass Designs in Three-High Shape-Mill Stands in Modern Mini-Mills

Construction and commissioning of a network of mini-mills of various maximum production volumes requires installation of equipment. Several plants have developed mills in which continuous-reduction stands have been replaced by three-high stands. Despite prior experience in using continuous-cast billets for production of a variety of rolled shapes in modern continuous mills, use of this approach in shape mills with three-high reduction stands requires an technical feasibility assessment of the new forming modes and pass designs used for these modes. Comparative analysis indicated that the newly developed billet forming modes and pass designs did not fully achieve the requisite technical feasibility. The stress-strain behavior in the early passes facilitates not only formation of end cracks, but also increases the likelihood of discontinuity defects in the metal over the entire free surface of the billet not in contact with the bottom or corners of the pass due to development of tensile stresses. Absence of reduction in the initial box passes during rolling of continuous-cast billets therefore will lead to rhombicity defects. Using excessively high reductions in individual passes makes clamping of the process cross section less stable, increases the likelihood of overfilling the pass, and increases the peak rolling load in certain passes Collectively this suggests that these process characteristics indicate that a better-validated approach is needed for determination of continuous-cast billet rolling schedules for three-high reduction stands.